The demand for hot water from a hot water system may vary considerably during the day. For a domestic system, there will be long period where no hot water is being drawn interspersed with much shorter periods where hot water is demanded, for example for showers or baths. Generally speaking, two alternative approaches to providing hot water are taken. The first approach, as shown in FIG. 1, is to use a boiler 2 to heat a tank of water 4 via a heat exchanger 6. Thus a relatively low capacity boiler is able to heat a reservoir of water within the tank 4 to an acceptably high temperature. When a user wishes to use the water, for example to run a bath, hot water is drawn off through an outlet pipe 8 at the top of the tank and cold water 10 is admitted to the bottom of the tank. Typically the cold water 10 comes from a separate header tank although in principle it can also come from direct connection to the cold main supplying the dwelling. In a domestic installation the boiler 2 may also have a heating hot water outlet and heating water return pipes 12 and 14, respectively, such that the boiler can heat the dwelling via a radiator system.
An alternative approach which is also common in domestic hot water and heating systems is the combination boiler as shown in FIG. 2. Here the store of preheated water is dispensed with and instead, when it is desired to use hot water, cold water is received by the boiler 20 directly from the cold water main 22 and is heated, in real time, within the boiler and output at a hot water outlet 24. The combination boiler 20 also has a heating water outlet and heating water return 12 and 14.
Each system has its own advantages and disadvantages. The system shown in FIG. 1 provides a plentiful supply of hot water, but once the water in the tank has been used, or rather exchanged with cold water, then there is a considerable delay before the water in the tank gets reheated to an acceptable temperature. The combination boiler system shown in FIG. 2 provides instantaneous supplies of hot water, but the flow rate of hot water is typically considerably restricted compared to the arrangement shown in FIG. 1.
These systems are also used on a commercial scale, for example in hospitals and leisure centres. In such arrangements there is generally a background level of substantially constant (mean) hot water usage, but otherwise similar considerations apply. Therefore, in order to satisfy the peak demand that is likely to be expected either large storage vessels are required such that the water in them can be heated when the boiler has a spare capacity to do so, or alternatively the boiler must be rated for the maximum expected demand and hence a larger and more expensive boiler system is required which generally runs at below its peak capacity.